Gas turbine engines include a compressor that compresses air, a combustor that ignites the compressed air and a turbine across which the compressed air is expanded. The expansion of the combustion products drives the turbine to rotate, which in turn drives rotation of the compressor.
In order to increase efficiency, a clearance between the tips of the blades in the compressor and the outer diameter of the flowpath is kept sufficiently small. This ensures that a minimum amount of air passes between the tips and the outer diameter. Some engines include an abradable outer air seal to further reduce tip clearance. The tips are designed to, at times, rub against the seal. The rubbing wears the abradable material of the seal. The tips have a reduced tip clearance relative to the idealized geometry formed in the abradable surface, thereby increasing efficiency.